An arrangement typical of existing endoscopes is shown schematically at 10 in FIG. 1. The laser 12 provides the excitation light for a fluorescent dye with which a sample has been stained or for reflection from the sample. A light separator in the form of coupler 14 couples laser light from the laser 12 to both endoscope head 16 and a power monitor 18. The power monitor 18 includes a power monitor device, and allows the operator to obtain some measure of the power being delivered to the endoscope head 16. The coupler 14 also couples the light returned by the sample (from low level excited fluorescence and/or reflection) back to a detection unit 20, which contains a barrier filter and a photomultiplier tube (not shown) to detect this return light. The core of optical fiber 22 between the endoscope head 16 and the coupler 14 is the transmission medium for both the excitation light and the return signal but, in some prior art systems, also constitutes a spatial filter (or, in effect, a pinhole) so that confocal detection may be effected.
In such arrangements, the laser 12, coupler 14, power monitor 18 and detection unit 20 are provided in a control box 24. Optical fiber 22 is detachably coupled to the control box 24 by means of an optical connector (not shown), so that the endoscope head 16 can be detached, such as for cleaning, between successive patients, or the like, by detaching the single optical connection between the control box 24 and that portion of the endoscope (i.e. optical fiber 22 and endoscope head 16) outside the control box.
Such arrangements have a number of problems. The relative power between the endoscope head 16 and the photodiode of the power monitor 18 is dependent on any joint loss at the connection between control box 24 to optical fiber 22 (due, for example, to dust in the connection). This means that the power monitor 18 needs, in principle, to be calibrated each time a new or replacement endoscope head 16 is connected. Where a 488 nm laser is used, it is necessary to ensure low loss at this interface for both the 488 nm excitation light and the 488 to 585 nm return light, as light of all these wavelengths is carried by optical fiber 22; the connection is between single mode fiber and requires low loss in both directions.
Further, this connection can affect the mode mix and hence the optical power distribution in optical fiber 22, which has the potential to produce image instability, noise and reduced resolution. In addition, reflection from this connection can cause instability in laser 12 resulting in image noise, and increase noise in the ultimate image by reflecting excitation light into the detection unit 20.